1) Field of the Invention
The invention relates to a device for laying a molten film emerging from a slot die onto a rotating take-off roll, having at least one electrode which is arranged parallel to and at a short distance from the surface of the take-off roll and to which electric voltage is applied.
2) Prior Art
In the production of films, with thicknesses between 0.5 .mu.m and 350 .mu.m, plastic granules are first melted in an extruder, and the extruded plastic melt is fed to a slot die. The prefilm emerging from this slot die is laid onto a take-off/chill roll and, after leaving the take-off roll, is stretched in and transverse to the machine-running direction in a stretching frame.
In order to stabilize the laying of the prefilm onto the take-off roll, in the prior art, additional forces (beyond the force of extrusion) are applied to the prefilm in the region between the emergence of the prefilm from the slot die and the point at which it is laid on the take-off roll. The additional forces can be produced by: using an air jet from an air knife to blow against the prefilm; spraying water into the gore of the prefilm before the point at which it is laid on the take-off roll; forming a water film on the take-off roll; and generating an electric field by means of a high voltage electrode, the field lines of the electric field being directed toward the surface of the take-off roll. The additional forces are used for the lateral outward displacement of air entrapped between the surface of the take-off roll and the underside of the prefilm, (the side of the prefilm facing the take-off roll), in order to stabilize the laying of the prefilm onto the roll surface over a predefined wrap angle. If this is not successful, air inclusions of so-called "pinning bubbles" are produced, which are located between the underside of the prefilm and the surface of the take-off roll. These pinning bubbles impair the smoothness and thus the quality of the prefilm cooled on the take-off roll, as well as the film biaxially oriented therefrom.
It is known to arrange parallel to the take-off roll surface a metal strip made of stainless steel which acts as an electrode. The metal strip is generally stamped out or cut out from a metal foil of about 20 .mu.m thickness in a width of 10 mm. A metal strip of this type has the disadvantage that it cannot absorb sufficiently high clamping forces, which are necessary to clamp the metal strip with the desired accuracy at a uniform distance from the take-off roll. Furthermore, it is disadvantageous that the edges of the metal strip produced by the cutting or stamping process are irregular, with the result that the electric field between the metal strip and the take-off roll is distorted. These distortions of the electric field are also further amplified because of irregular spacing of the metal strip from the take-off roll surface. The distortions of an electric field lead on the one hand to electric flashovers or arcing, by which the prefilm can be damaged, and on the other hand to the prefilm being laid non-uniformly onto the take-off roll, as a result of which air inclusion between the prefilm and the take-off roll can continue to occur. The distortions of the electric field overall do not lead to the de sired film quality.
The PCT Application WO 95/21735 (German Utility Model G 94 02 027) discloses a device for the production of films, in particular thin or very thin films, having a rotating roll, using a plastically deformable film, preferably supplied through a slot die. The film is extruded or otherwise laid onto the chill roll and an electric voltage is applied to the film between the roll and a metal strip that is arranged parallel to and at a short distance from the roll. The metal strip has a sharp longitudinal edge which is arranged next to and is directed toward the roll at a distance smaller than 6 mm. The sharp longitudinal edge is constructed in the manner of a cutting edge on a sharply tapering region of the metal strip. The center line of the sharply tapering region is aligned with the center of the roll. The use of such a metal-strip electrode improves the film quality, since the air inclusions become smaller in diameter and fewer in number, but not to the extent desired.
U.S. Pat. No. 5,494,619 discloses an electrostatic pinning process in which a wire electrode is arranged between an extrusion die and a take-off roll. A shield at bias voltage is located between the extrusion die and the wire electrode. The bias voltage of the shield is 1 to 10 kV, whereas a voltage of 6 to 15 kV is applied to the wire electrode. In this process, it is essential to apply the highest possible electrostatic charge to the film to be laid onto the take-off roll, in comparison with an arrangement in which the shielding is at ground potential. To this end, there is an optimum bias voltage of the shield, which depends on the voltage of the wire electrode, but differs significantly from the latter and from ground potential. The net current or the electrostatic charge to the film results from the difference between the current from the wire electrode and a previously measured current to the shield. The measurement of current to the shield is obtained by applying voltage to the shield with the wire electrode being at ground potential. For a predefined voltage, which is applied to the wire electrode, a high current is generated from the low electric voltage on the shield having a great number of field lines. The majority of the currents remains on the shield and not on the film. By increasing the voltage on the shield, the current decreases, but more field lines end on the film, and the major part of the current is deflected onto the film. The overlaying of two electrostatic fields does not take place, since the electrically biased shield serves to deflect field lines directed toward it from the wire electrode in the direction of the film.
The object of the present invention is to develop further and to improve a device of the type described in such a way that the air inclusions become significantly smaller and fewer than in the case of devices for films known in the prior art.